Thanks to Isarra's patch on T37337 (https://gerrit.wikimedia.org/r/195195) we now can set HD site logos. The configuration, entirely reasonably, only supports 1.5x and 2x pixel densities. However, when using a 1.25x screen (or zooming in on normal screen), the 1x logo is displayed; I think it would be better to display the 1.5x one, scaled down.
Some mid-end Windows 7 and Windows 8 laptops use 125% DPI by default. I think it's worthwhile to support them in this regard. (Alas, I couldn't find any hard data on what kind of screens this affects and how many there are, although there's plenty people reporting this over the internet; I can provide a data point of my 15" laptop with 1680px horizontal resolution being this way.)
The current code uses 1.5x image for DPIs >=1.5x, and 2x image for DPIs >=2x; can we instead do it for >1x and >1.5x, respectively?